Drive circuit



United States Patent 3,466,507 DRIVE CIRCUIT Robert A. Ragen, Hayward, Calif., assignor to Friden, Inc., a corporation of Delaware Filed June 2, 1966, Ser. No. 554,811 Int. Cl. H0111 47/32 US. Cl. 317148.5 4 Claims ABSTRACT OF THE DISCLOSURE A drive circuit is disclosed having a storage capacitor which is charged by potential source through a constant current regulator and discharged through an inductive load to a reference potential. A unidirectional conducting device is connected between the load and the potential source in order to dissipate the current flowing through the load after the discharge path is interrupted. One such inductive load comprises a coil disposed about a magnetizable core which actuates a pivotable print hammer when the coil is energized. The discharge circuit is adapted to be deactivated in such a manner that the hammer smartly strikes its target in a nonrepetitive fashion.

This invention relates to a drive circuit and, in particular, to a solid state circuit for driving a print hammer utlized in high-speed printer apparatus.

Current technology imposes stringent requirements for high speed in the processing of information or data. This is especially true when providing readout equipment for storage computers. Since computers generally operate at electronic speeds, and printers are basically electromachanical in nature and thus slower in operation, continuous efforts are being made to increase the speed of operation of such printers.

One element of a high-speed printer which is of particular concern is the print hammer. Prior art print hammer drive circuits generally employ electromagnetic actuater, including a coil, to urge the hammer forward to strike a paper target for imprinting. The coil receives energizing current from a discharging capacitor, which is coupled to a power supply through a resistance. Generally, the charging current for the capacitor is a very high peak current, which unduly loads the power supply. Also, in such known circuits, when the drive pulse to the coil is terminated, the change of current in the coil is opposed by the reverse voltage developed by the coil, thus prolonging the fall of the coil current to zero value. Consequently, the hammer may be retarded in its return to home position, or may be activated to move forward again prior to returning to its home position. Thus, the printer apparatus may experience smearing or spurious repetitive striking of the hammer at a single character position, among other things.

An object of this invention is to provide a novel and improved drive circuit for a print hammer assembly.

Another object of this invention is to provide a drive circuit that utilizes substantially less peak power than prior known drive circuits of this type.

Another object is to provide a hammer drive circuit for a high-speed printer apparatus wherein erratic printing is effectively eliminated.

According to this invention, a drive circuit useful with a print hammer of a high-speed printer comprises an electromagnetic actuator, including an inductive coil, which is energized to cause the hammer to move rapidly from a home position to a striking position. The coil receives an energizing current pulse from a discharging capacitor that is coupled to a power supply through a constant current regulator. The current required for operation is held to a value determined by the current Patented Sept. 9, 1969 ice regulator and as a result peak power requirements are greatly reduced. Also, the inventive drive circuit includes a unidirectional conducting device for causing current in the coil to swiftly fall to a negligible value when the drive circuit is opened, thereby affording a rapid, uninterrupted return of the hammer from the target to home position.

The invention will be described in greater detail with reference to the drawing in which:

FIG. 1 is a schematic diagram depicting the novel drive circuit, as used with a print hammer; and

FIGS. 2A through 2C are plots of current voltage against time, contrasting the performance of prior art drive circuits with that of the present inventive circuit.

With reference to FIG. 1, the drive circuit comprises a constant current generator which consists of negative voltage source 26 and constant current regulator 10. Regulator 10 includes a PNP transistor 12, a Zener diode 14, a resistor 16 coupled to the base 18 of the transistor 12, and a resistor 20 connected to the emitter 22 of the transistor 12. The collector 24 and one end of the resistor 16 are coupled to a source of negative potential 26; whereas the emitter circuit of the transistor 12 and the cathode of the Zener diode 14 are coupled to a source of reference potential, such as ground, through a capacitance 28. The transistor base 18 is connected to the junction of resistor 16 and the anode of the Zener diode 14. The transistor emitter circuit and capacitance 28 are coupled to one end of the coil 30. The other end of the coil is coupled to the collector of switching transistor 38 and to the cathode of di-ode 32. The emitter of transistor 38 is connected to ground; the anode of diode 32 is connected to voltage source 26.

During the charging period, capacitor 28 charges linearly through constant current regulator 10 to a value determined by the voltage source 26 and the parameters of the regulator. As shown in FIGS. 2A and 2C, the charging current I is maintained at constant value until the capacitor is fully charged, at which time the current drops to zero value. Since the charging current is constant, loading of the source 26 remains constant during the charging cycle. This is in contrast to prior art systems utilizing a voltage source charging a capacitor through a resistor, in which the charging current I (FIG. 2A) rises quickly to a maximum value and then decreases exponentially, thereby causing heavy loading of the source over a large portion of the cycle and a consequent heavy power consumption. During the charging of the capacitor 28, switching transistor 38 is turned off and diode 32 is reverse biased so that no current will flow through the coil 30.

. In order to actuate the load which, in this case, is a print hammer 34, a pulse of negative polarity is applied to the base terminal 36 of a grounded emitter PNP transistor 38, biasing the transistor 38 into conduction. The negative pulse may be a square wave of short duration, developed by depression of a key in a printer keyboard, by way of example. When the transistor 38 conducts, a current path including the coil 30 is established, resulting in the discharge of capacitor 28 through the coil 30 and transistor 38 to ground. During this interval, the current in the coil at first rises swiftly to a peak value, quickly drops, and then approaches a limiting value, which is the value of the current supplied by the constant current generator. If switching transistor 38 were left on for a relatively long period of time, the current through coil 30 would assume this value.

During the interval of current flow though the coil 30, a core 40, encompassed by such coil, is energized, which magnetically attracts one end 43 of the steel print hammer 34. The hammer rotates about a pivot 42 in response to the attractive pull of the solenoid core 40. As a result, the other end or head 44 of the print hammer 34 is proficient to strike its target, which may be a paper or other record medium disposed adjacent to a character support or other data imprinting device.

Upon termination of the negative pulse to the base ofswitching transistor 38 (or upon application of a pulse of positive polarity), this transistor is turned off which opens the path between coil 30 and ground. This causes the current through the coil to begin to decrease very rapidly. This rapid decrease is opposed by the coil and a reverse voltage of extremely high magnitude develops across the coil. Since capacitor 28 is at nearly ground potential as shown in FIG. 2C wherein V represents the magnitude of the capacitor potential, the end of coil 30 which is connected to the cathode of diode 32 assumes a negative potential much lower than the potential of source 26. This forward biases diode 32, thereby providing a conductive path through the diode to source 26 for the aforementioned rapidly decreasing current in the coil. This path remains open so long as the end of coil 30 which is connected to the cathode of diode 32 is at a lower potential than source 26. This remains true so long as the current in coil 30 is decreasing rapidly. Thus, the opposition to the reduction of current flow in the coil is effectively removed, allowing the current in the coil to fall swiftly to zero, as shown in FIG. 2B, wherein I represents the coil current. When the rate of change of coil current becomes zero, the potential across the coil becomes zero which reverse biases diode 32, thereby opening this current path. When this occurs (at time t the current in the coil also becomes zero and the capacitor 28 begins to charge again.

This method of damping the coil current after turning off the switching transistor 38, wherein an alternate path including the diode is provided for current to flow, substantially dissipates all the energy in the coil 30, whereby the hammer 34 is no longer attracted to the core 40. The damping technique disclosed herein takes place in a much shorter time than that achieved by previously known methods, as can be seen from FIG. 2B. This damping occurs before the lower end 43 of the hammer reaches the core 40. Thus the solenoid is de-energized and the force of attraction is terminated while the hammer is still pivoting (clockwise as shown) with*the hammerhead 44 moving forward in the direction of the target. The head 44 of the hammer nevertheless maintains its forward motion toward the target by virtue of its momentum. The current in the coil 30 is substantially at zero before the hammer strikes the paper, so that the hammer does not dwell on the paper. This eliminates smearing and a multiple impression of the imprinted character.

Immediately after the head 44 contacts the target, a resilient means or spring 46 acts to retract the head of the The drive circuit of this invention provides energy for urging a print hammer forward to impinge on the paper target, and also to rebound from the paper upon impact. Since substantially no current flows in the coil 30 at the instant of the impact, the hammer may return freely to its home position. In addition, the print hammer can'be retracted rapidly so that its action is compatible with the the high-speed operation of the printer apparatus.

In a successful embodiment of the novel circuit, typical values of circuit elements were as follows:

Transistor 12 2N669 Transistor 38 2N1358 Resistor 16 ohms 330 i Resistor 20 do 22 Resistor 52 do 22 Capacitor 28 ,u.farads 120 Capacitor 50 do 0.1 Coil 30 millihenries.. 5.6 7 Zener diode 1N709 hammer freely to reset position, from which the hammer the pivot point than the hammer end 43 which is located in juxtaposition to the solenoid 30. After the hammer has rebounded to rest or normal position, the capacitor 28 continues to charge, and the hammer is ready for the next actuating signal to print.

The drive circuit also includes a capacitor 50 and re sistor 52 that serve as an R.F. suppress-or, which minimizes noise and transient effects.

The drive circuit of this invention allows a substantial reduction in peak power, in the order of 5:1, for example. In a successful embodiment of this invention, power requirements for charging a capacitor used in a hammer drive circuit of a high-speed printer were reduced from 1 ampere to 200 milliamperes approximately.

It is understood that the invention is not limited to the particular values or parameters set forth above, and that modifications may be made within the scope of this invention.

What is claimed is:

1. A print hammer drive circuit for intermittently driving an inductive load to cause a print hammer to strike a target in nonrepetitive fashion comprising:

a storage capacitor;

a constant current generator means for'charging said capacitor at a constant rate comprising a source of electrical potential and a constant current regulator coupled between said source and said capacitor;

an inductive load comprising a coil disposed about a magnetizable core;

a pivotable print hammer responsive to energization of said coil, said print hammer having a head end and a core end;

means coupled to said capacitor for discharging said capacitor through said load at desired intervals, said discharging means including a switching transistor coupled between said inductive load and a reference potential and adapted to be deactivated before said head end strikes its target; and

means coupled to said load for dissipating the current in said load before said head end strikes its target upon deactivation of said discharging means, said dissipating means comprising a unidirectional conducting device coupled between said load and said source for enabling current to flow from said load to said source when said discharging means is deactivated.

, 2. The circuit of claim 1 wherein the collector of said switching transistor is coupled to one end of said load and the emitter of said switching transistor is coupled to said reference potential.

3.'The circuit of .claim 1 further including means for biasing said head end of said print hammer away from said target.

4. The circuit of claim 3 wherein said biasing means comprises a spring coupled to said print hammer.

References Cited UNITED STATES PATENTS LEE T. HIX, Primary Examiner U.S. c1. X.R. 317 1s1, 320-1 

